Study On Sintering Characteristic And Piezoelectric Properties Of Sodium Niobate Based Lead-free Piezoceramics

In this dissertation, the relationship among composition, manufacturing process, microstructureand properties of fine grained （K, Na）NbO₃-based piezoelectric ceramics have been systematicallyinvestigated by X-ray diffraction （XRD）, scanning and transmission electron microscopy （SEM andTEM）, differential scanning calorimeter（DSC）, and Raman Spectroscopic Analysis.The valence-electron structures （VES） of sodium-potassium niobat(eK_xNa_1-xNbO₃)were calculated,and the relationship between the VES and the piezoelectric properties was determined. The resultsindicate that the piezoelectric constants are continuous and equal in the composition range of0.4≤x≤0.5. The piezoelectric properties do not dramatically improved at the so-called MPB. Theexperimental results show that the change trend of the piezoelectric constants is in good agreementwith the calculation results. Contrary to traditional MPB, the so-called morphotropic phase boundaryof sodium-potassium niobate has no significant influence on the piezoelectric and ferroelectricproperties. In other words, it was speculated that the existence of the traditional morphotropic phaseboundary was possibly controvertible in the KNN system.The influence of the ZnO, CuO, and K_5.70Li_4.07Nb_10.23O₃₀（KLN） as sintering aids on themicrostructure and properties has been studied. Our results reveal that a small amount of ZnO canimprove the density of the ceramics effectively. Because of the high density and ZnO doping effects,the piezoelectric and dielectric properties of the ceramics are improved considerably. Some abnormalcoarse grains were formed in a matrix when the content of KLN was relatively low1mol%. Propercontent of KLN decreased the amount of defects, thus the remnant polarization increased and thecoercive field decreased markedly, and the sinterability of the KNN ceramics was simultaneouslyimproved with significant increase of piezoelectric properties. The mechanical quality factor Qmcouldsignificantly be improved for “hardening” effect of the Cu2+-doped. For the ceramics with1.5mol%,the best performance of piezoelectric ceramic was obtained.Low temperature sintering of （K）（0.5）Na_0.5)NbO₃was investigated using Bi₂O₃and Li₂CO₃assintering aids. The optimal piezoelectric properties are obtained for the ceramics sintered at900oC for5h. The dielectric constant （ε）, piezoelectric coefficient （d33）, and electromechanical couplingcoefficient （kp） show peak values of877,92pC/N, and0.27, respectively. These values arecomparable to the values obtained for （K）（0.5）Na_0.5)NbO₃ceramics sintered above1100oC.The sintering process and mechanism of （K, Na, Li）（Nb, Ta, Sb）O3（KNLNTS） solid solution were investigated. The results show that the KNLNTS forms via the reaction of A₂CO₃（A: K、Na、Li） andB2O5（B: Nb、Ta、Sb） at400-800℃. Based on the reaction kinetic isothermal analysis, KNLNTSformation is corroborated as being controlled by diffusion mechanism. The sintering process in theKNLNTS solid solutions may be explained by the grain boundary diffusion model.Microstructure characteristics, phase transition, and electrical properties of (K_0.4425Na_0.52Li_0.0375)(Nb_0.8925Sb_0.07Ta_0.0375)O₃（KNLNST） lead-free piezoelectric ceramics were investigated with anemphasis on the influence of sintering temperature. Orthorhombic phases mainly exist in the ceramicssintered at1100–1130℃, whereas the tetragonal phase becomes dominant when sintering temperatureis above1130℃. Because of the existence of MPB-like transitional behavior, the piezoelectricproperties show peak values at1120℃.Two-step sintering was investigated to manufacture (K_0.5Na_0.5)NbO₃-based lead-free piezoelectricceramics. Under the optimal condition, dense specimens have an average grain size of approximately5μm, and show good dielectric and piezoelectric properties. Furthermore, the (K_0.5Na_0.5)NbO₃-basedceramics maintain relatively low dielectric loss over wide temperature ranges.The effects of the Ta and Sb content on the phase structure, microstructure, dielectric, piezoelectric,and ferroelectric properties of the (K_0.4425Na_0.52Li_0.0375)(Nb_0.9625-xSb_xTa_0.0375)O₃ceramics wereinvestigated. By increasing x from0.05to0.09, all samples exhibit a single perovskite structure withan orthorhombic phase over the whole compositional range. The grain growth of the ceramics wasimproved by substituting Sb5+for Nb5+. The (K_0.4425Na_0.52Li_0.0375)(Nb_0.8925Sb_0.07Ta_0.0375)O₃ceramicsshow the peak values of the piezoelectric properties. By increasing x from0.03to0.0675, thepiezoelectric performance appears regular change over the whole compositional range, owing toimprovements in d33, kpand a decease in Q_m. Curie temperature TCshift toward lower-temperatureregions by increasing the content of Ta, and Ta would weaken the ferroelectricity of the ceramics. Theoptimal performance of piezoelectric ceramic was obtained at x=0.0375.